In general, the active material in the positive plates of an alkaline storage battery is nickel hydroxide. The active material in the negative plates of such an alkaline storage battery is usually cadmium or iron in the charged state. Regardless of the active material in the negative electrodes, alkaline storage cells are frequently referred to as steel accumulators. This reflects the fact that iron is used as a constructional material in the storage cell. Thus, not only are the negative plates made from iron, but also the positive nickel electrodes are constructed using a perforated nickel-plated steel strip as the support for the active material. The nickel-plated steel strip adds mechanical sturdiness to the nickel electrodes and also acts as a mass carrier.
Tubular electrodes, first developed by Edison, and which have hardly changed since then, also employ a steel strip or ribbon. The steel strip in such tubular electrodes is wound spirally on the mandrel of a drawing machine. These tubular electrodes are folded at their ends and contain within them alternating layers of nickel(II) hydroxide and nickel flakes as the conducting material.
Positive sinter electrodes also employ a very thin, perforated steel strip. On both sides of this steel strip, nickel powder is pasted on and sintered in a continuous process. Incidentally, negative cadmium sinter electrodes are constructed in a similar manner.
One of the problems with prior art alkaline storage batteries arises from the deleterious effects of iron impurities. If one disregards the contribution from the iron/iron hydroxide negative electrode of an Edison storage battery, the main source of iron impurities in alkaline storage batteries is the steel strip. These iron impurities have a harmful effect on the operation of the positive nickel electrodes. Iron dumping arises in the areas of the steel strip surface where there are pores, or in areas of the steel strip which are damaged. Iron dumping can reach a substantial extent, especially in the case of electrodes having fiber metal frameworks which are becoming increasingly popular.
S. U. Falk and A. J. Salkind, in Alkaline Storage Batteries, John Wiley & Sons, Inc., New York, London, Sidney, Toronto (1969), at pages 630-631, suggest that the harmful iron impurity in an Edison-type storage battery arises from iron that is dissolved by anodic oxidation in KOH to form primarily ferrate ions. The ferrate ions are not stable but decompose spontaneously into iron(III) hydroxide and oxygen.
The harmful effect of the iron is seen in a decrease of the charging efficiency of the nickel electrode. This detrimental effect is believed to result from a reduction of the oxidation state of the nickel hydroxide to a state which does not correspond to a full charge which is caused by the fact that the iron hydroxide reduces the oxygen overvoltage at the surface of the electrode.
In reality, the harmful effect of iron can be attributed to colloidally dissolved iron aquoxide particles, which can be demonstrated to be present in such electrolytes by analytical means and by the Tyndall effect. Such particles are drawn into an inhomogeneous magnetic field (of the kind surrounding any electrode passing current), because of a negative surface charge on such particles, and also, as W. Bohnstedt et al., at 19 Electrochem. Acta, 941 (1974), have shown, because of the super-paramagnetic behavior of such particles. During the charging process, the iron aquoxide particles concentrate near the anode and an electro-phoretic dissolution occurs at the surface of the nickel hydroxide.
In accordance with these findings, Mlynarek et al., at 14 J. Appl. Electrochem, 145 (1984), have suggested that the potential range which is achieved during charging of the nickel hydroxide electrode is sufficient to form magnetite (Fe.sub.3 O.sub.4). They view this magnetite formation to be the cause of the evolution of oxygen, which they proved to occur by volumetric methods, during charging of the nickel hydroxide electrode in the presence of iron impurities. As is well known, magnetite is a good current conductor and exhibits a small oxygen overvoltage.
Heretofore, the nickel electrode could only be protected from the harmful effects of colloidal iron aquoxide particles by employing dense membrane separators. However, the use of such membrane separators is unsatisfactory because of their high production costs. In addition, because of their relatively high resistance, the membrane separators substantially limit the maximum charge and discharge current densities which can be achieved.